This invention relates to surface acoustic wave devices and, more particularly, to the fabrication of a grating which may be used in bandpass filters.
Gratings have been utilized in surface acoustic wave (SAW) devices for reflecting surface waves back and forth, as in a resonator, or for directing such waves along a path from an input transducer of the device to an output transducer thereof. Such gratings are fabricated typically by a set of discontinuities in the surface of the device, the discontinuities having a form of a set of strips. For example, such strips may be cut into the surface of the device to a depth a small fraction of a wavelength or may be formed by material deposited on the surface of the device to a depth of a small fraction of the wavelength of sound propagating along the surface of the device. Individual strips reflect no more than a small fraction of the total acoustic energy in the surface wave so that, when it is desired to reflect all or nearly all of the energy in the surface wave, a grating of many strips is utilized.
A problem arises in that the use of a grating in the form of an array of many parallel strips produces a radiation pattern, such as that of an end-fire array of sonic radiators, wherein the radiation pattern of the grating is sharply defined over only a relatively small bandwidth of frequencies of the surface wave. Gratings of fewer strips have a broader bandwidth but suffer from a diminution in the total reflected energy.